1. Field of the Invention
This invention relates to electrography, and, more particularly, to an electrographic printing system for producing an image on a dielectric film member.
2. Description of the Prior Art
The art has recognized that images can be reproduced on a dielectric surface of an image member through various electrographic processes. In such processes, an electrostatic latent image is generated on the dielectric from a metallic electrode by air ionization. Usually, the electrostatic latent images are generated by character shaped stylus or pin electrodes which are brought into close proximity to a dielectric web supported on a base electrode. In such processes, a potential is applied across the electrodes below a critical stress value. Then transfer of the character or pin configuration from the electrode to the insulating web is effected by the use of a relatively low potential triggering or signal pulse which raises the electric field above a critical stress value required to produce a field discharge in the space between the insulating web and the base electrode. The discharge action gives rise to the formation of an electrostatic latent image of the character or pin on the insulating web. Thereafter, the generated image on the insulating web can be rendered visible by application of liquid or dry developer thereto, and fixed into a permanent image on the dielectric record member.
Electrography is useful in many applications where it is required that a voltage signal pulse be applied directly to a dielectric receiving member, e.g. in analog oscillographs, high speed line printers, digital plotters and the like.
In these systems, the dielectric record member was invariably dielectric-coated paper, which is prepared by impregnating a paper base or substrate with a conductive material and then coating the dielectric layer thereon. Apparently paper was used as the substrate material of the dielectric receiving member because it was believed that the substrate had to possess volume conductivity, that is, the base itself had to be conductive in order for the system to be operative. Thus the art employed paper only as the base material because paper could be easily impregnated through its thickness of the paper with a conductive material to achieve the desired volume conductivity in the base layer.
While such dielectric-coated papers are useful in these applications, it would be of considerable advantage to provide an electrographic printing system in which the electrostatic image is formed on a dielectric film record member. Films offer many advantages over paper since they can be handled easily, are durable, have a long shelf life, and multiple copies can be made readily. Most importantly, images formed on a transparent film member could be projected onto a screen and viewed by many persons simultaneously. These films would also permit overlay examination. The criteria for usefulness of such film record members in these applications, however, is that the images must be clear, sharp, of high-contrast, stable, and be available on a transparent base.
Accordingly, it is an object of this invention to provide an electrographic printing system in which electrostatic images are formed directly onto record members other than paper, and, more particularly, onto plastic film record members.
Another object of this invention is to provide an electrographic printing system in which such a film record member can be transported rapidly across character-generating electrodes to form clear, sharp images.
Still another object of this invention is to provide an electrographic printing system in which permanent images can be fixed into a dielectric film record member as clear, sharp images of high contrast, which may be viewed readily in an overhead projector.
In accordance with these objects, an effective electrographic printing system is provided in which the base of the dielectric imaging member is a plastic film on which is formed a layer of a conductive material, and including a dielectric layer coated on the conductive layer. The film record member of this invention is a three-layered structure, in which the base is plastic, instead of paper, is not conductive itself, and a conductive layer is sandwiched between the base and the dielectric layer. Another feature of this film member in the system is that the thickness of the dielectric layer is about 1.4-10 micrometers.